Question How to properly set up rain splashes

Hello. First thing that I would do is to take a look at the Scene Depth to take a look at who is writing into it. You do this by going into Window -> Analysis-> Rendering Debugger -> Material Tab -> Gbuffer -> Depth
By doing so, you should be able to visualize the depth to be sure that no unwanted Meshes or particles are writing to the Scene Depth.

• If everything is looking good, you can check for the lifetime of our particles. If your particles have lifetimes, it might not be long enough for them to hit the Surface. In this case they will die before hitting the surface and so If you're passing this position to their "child" particles the position won't be on the surface. In this case, try to increase the lifetime, see if it makes some differences.
  
  In this example, as some particles have a small lifetime of 1, they're dying before colliding with the Scene Depth.
  
  
• This can also be caused by some precision issues. If the speed of your particles is very high, this can lead to this can of problem. What you can do to verify this is simply by drastically lowering the speed of your particles to see if the precision of your collision is coming from this factor.
  
  In this example, as the Gravity Force is increased, the precision issue start to occurs.
  
  Now, the solution to solve your issue will depend on what you're trying to achieve... Do you need your scene to be dynamic ? If you just need to collide on one mesh and or a terrain, there are alternatives to using the scene depth that would unlock other options. Signed Distance Field can be an interesting alternative that might give you better results. When dealing with this kind of precision issues, what I often like to do is to "reverse" the spawning. So when doing rain falling, or leafs etc...
  What you can do is spawn The Rain Drop from the Collision surface/terrain, basically where they will die. You store this "Initial Position" and offset it on the World-Y axis. This way you can still use collision and spawn new particles but instead of using the "collided" position which can be "imprecise" you use the "Initial Position" stored before. You can even skip the "collision pass" by simply "lerp/interpolate" over the particle's lifetime between the "Initial Position" and the "Y-offset position"
  
  
  To Finish, I would even say that for this kind of effect if the Rain intensity is pretty high, you can even create two separate system without the need of the Trigger Events. You would not have a perfect correlation between the Droplet and the splash, but the eyes won't be able to notice....if there are a lot of high speed particles and that your camera isn't going to be doing extra close up, you can save performances by not using trigger events.
  
  Hope this will help you find why your particles are not properly positioned. After this will be time to try to find the best solution to resolve or mitigate this issue. Have a great deay

 

Even thought the Events are running on the GPU, they still have a cost. You can have some "congestion" issue if a lot of particles are dying at the same frame because that would mean a lot of Trigger events at the same frame.
Also, GPU Event means that you can't have instancing of your VFX.

In the end, it's always about faking or giving the illusion to the player/viewer. So in your case, if your camera isn't doing close-up shots and your rain is intense enough, no one will notice that the drop and Splash aren't in sync.
That being said, Trigger events are perfectly fine to use
Just need to be figured out if you really need this accuracy, if not save some performances and off course spend it elsewhere.